June Key Lee. Department of Materials Science and Engineering, Chonnam National University, Gwangju (Received 26 August 2008)

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1 Journal of the Korean Physical Society, Vol. 55, No. 3, September 2009, pp Surface and Electrical Properties of Inductively-coupled Plasma-etched N-face n-gan and a Method of Reducing the Ohmic Contact Resistance of Plasma-damaged N-face n-gan Tak Jeong, Hyun Haeng Lee, Kang Ho Kim, Seong Ran Jeon, Seung Jae Lee, Sang Hern Lee and Jong Hyeob Baek Korea Photonics Technology Institute, Gwangju June Key Lee Department of Materials Science and Engineering, Chonnam National University, Gwangju (Received 26 August 2008) An investigation of the surface and the electrical properties of inductively-coupled plasma (ICP)- etched N-face n-gan is reported. Furthermore, a method for reducing the Ohmic contact resistance on plasma-damaged N-face n-gan is proposed. The I V characteristics show that the Cl 2 flow rate used for the ICP etching has a strong influence on the Ohmic behavior of the etched N-face n-gan. We also found that the surfaces of the ICP-etched N-face n-gan were damaged by increasing the Cl 2 flow rate, resulting in deteriorating Ohmic behavior. However, the plasma damage caused by the ICP etching could be removed by using a KOH etching process, which resulted in a decreased contact resistance. PACS numbers: Bn, Cg, Mr Keywords: Plasma damage, N-face n-gan, Inductively-coupled plasma (ICP), Ohmic contact resistance I. INTRODUCTION Currently, there is a great need for an improvement of the internal quantum efficiency and of the extraction efficiency in GaN-based light-emitting diodes (LEDs). There have been many reports on methods to improve the light extraction efficiency, such as the use of flipchip LEDs (FCLEDs) [1], the roughening of the top LED surface [2], and the introduction of highly reflective omnidirectional reflectors (ODRs) [3 8]. However, all of the devices mentioned above were realized in a conventional p-side up lateral structure. This configuration suffers from a severe current-crowding effect and heat-conducting problem due to the insulating sapphire substrate. Therefore, achieving high-power and highefficiency devices with a reduced current crowding effect and good heat dissipation is a challenging goal. At present, vertical-injection GaN-based lightemitting diodes (VLEDs) are being extensively investigated [9 13]. This type of diode was developed via the removal of the insulating sapphire substrate and the fabrication of contacts on both the top and the bottom sides. Compared with the conventional lateral structures, VLEDs have many advantages, such as jhbaek@kopti.re.kr; Fax: better current injection, good heat dissipation (due to the wafer bonding or electroplating using highly conductive materials), and enhanced reliability with respect to electrostatic discharge. In the fabrication of VLEDs, undoped GaN has to be removed to expose the n-type GaN layer after the laser lift-off (LLO) process. This exposed n-type GaN layer is formed to have the structure of an N-face polarity. The removal of the undoped GaN has generally been conducted by using an inductively-coupled plasma (ICP) etching system. ICP etching offers many advantages, such as high plasma density and the ability to independently control the ion energy. These allow higher etch rates and smoother surface morphologies to be obtained [14]. However, the plasma-induced damage that occurs during the ICP etching process destroys the surface of the GaN film and, thus, affects the contact resistance. Although there have been many reports on the effects of plasma-induced damage on the electrical and the structural characteristics of Ga-face n-gan [15 17], little research has focused on the effects of plasma damage on N-face n-gan. In this study, we have prepared N-face n-gan samples by using LLO, followed by a wafer bonding technique. We have also investigated the surface and the electrical properties of ICP-etched N-face n-gan. Furthermore, a method for reducing the Ohmic contact resistance on plasma-damaged N-face n-gan is proposed.

2 Surface and Electrical Properties of Inductively-coupled Tak Jeong et al II. EXPERIMENTS AND DISCUSSION The LED structures examined in the present work consisted of sapphire (0001) substrates (polished on both sides), 1.8-µm-thick undoped GaN, 2-µm-thick n- GaN, five periods of 14-nm-thick AlGaN/2.5-nm-thick InGaN multiple quantum well, a 10-nm-thick AlGaN electron blocking layer, 120-nm-thick p-gan, and a 4- nm-thick u-gan/ingan contact layer. For the device fabrication, ITO/Ag (5 nm/200 nm) was deposited as a reflector metal. This was then annealed at 550 C for 1 min. Then, an adhesive/barrier layer consisting of Ni/Ti/Pt/Ti//Ni/Ti was deposited by using an e-beam evaporator. An AuSn bonding layer was subsequently deposited on top of the adhesive/barrier layer. Next, the LEDs were bonded to the Si wafer, which had also been previously coated with an AuSn bonding layer. The sapphire substrate was then removed using a KrF laser beam (248 nm). All of the samples were dipped into a boiling HCl solution (HCl : DI = 1 : 1) for 2 min to clean their undoped GaN surfaces. The undoped GaN surfaces were then etched using an ICP etching system (Oxford, plasmalab 100) with various etching conditions to expose the n-gan layer. These will hereafter be referred to as the ICP-etched samples. The ICP power, bias power, and operating pressure were kept constant at 1000 W, 50 W, and 6 mtorr, respectively. The etching depth of the undoped GaN was adjusted to 2 µm for all of the samples in order to expose the n-gan surfaces uniformly. Various flow rates of BCl 3 /Cl 2 were used in order to determine the effect of the gas flow on the surface morphology. Following the ICP etching process, the samples were dipped into a buffered oxide etch (BOE) solution for 10 min to clean their surfaces. The surfaces of the ICP-etched samples were analyzed with scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS, MultiLab 2000 model). For the analysis of the Ohmic properties of the ICP-etched N-face n-gan, circular-transmission line model (CTLM) patterns were fabricated using photolithography and a liftoff process. Then, Cr/Ni/Au (20 nm/25 nm /400 nm) were deposited onto all of the ICP-etched CTLM samples. The current-voltage (I V ) characteristics were analyzed using a semiconductor parameter analyzer. Finally, all of the ICP-etched samples were etched again in a 5-M KOH solution at 85 C for 5 min. These samples were then rinsed in distilled water and dried in flowing N 2, and will hereafter be referred to as the KOH-etched samples. The surface properties and the I V characteristics of the KOH-etched samples were also investigated using CTLM patterns. The surfaces of the KOH-etched samples were also analyzed using AFM and XPS. Figure 1 shows AFM images of the as-grown and ICPetched N-face n-gan surfaces. The surface roughness of the ICP-etched N-face n-gan increased upon increasing Cl 2 flow rate. The rms (root mean square) values of the Fig. 1. AFM images of (a) the as-grown N-face n-gan and the ICP-etched N-face n-gan with BCl 3/Cl 2 flow rates of (b) 35/5, (c) 25/15, and (d) 5/35 sccm. Fig. 2. I-V curves of the Cr/Ni/Au contacts on the ICPetched N-face n-gan samples. surface roughness were 5, 27, and 310 nm for the samples etched using BCl 3 /Cl 2 flow rates of 35/5, 25/15, and 5/35 sccm, respectively. By comparison, the rms value of the surface roughness for the as-grown N-face n-gan sample was 2.7 nm. It is clear to see that the surfaces of the ICP-etched N-face n-gan samples rapidly deteriorated upon increasing Cl 2 flow rate. In addition, the etch rate was greatly increased with increasing Cl 2 flow rate. As is generally known, chlorine plays a major role in accelerating the etching by chemically reacting with the GaN. Therefore, the deterioration of the n-gan surface with increasing Cl 2 flow rate is mainly attributed to the higher Cl concentration and to the formation of GaCl x. On the other hand, the sample etched using the BCl 3 /Cl 2 flow rate of 35/5 sccm exhibited a smooth surface morphology, although the rms value was slightly increased compared to that of the as-grown N-face n-gan sample. We believe that this is due to the lower Cl concentration allowing the simultaneous reaction between Cl and GaN to form volatile products, such as GaCl 3. This results in

3 Journal of the Korean Physical Society, Vol. 55, No. 3, September 2009 Fig. 3. XPS spectra of the Ga3d core level for the ICPetched N-face n-gan samples. a homogeneous surface morphology. Figure 2 shows the I V characteristics of the Cr/Ni/Au Ohmic contacts on N-face n-gan samples that had been ICP etched at various BCl 3 /Cl 2 flow rates. These were measured between CTLM Ohmic pads with a spacing of 50 µm. As a reference, the as-grown N-face n-gan sample exhibited a highly Ohmic behavior, and the calculated specific contact resistance (R SC ) was determined to be Ωcm 2. However, for the ICPetched N-face n-gan samples, the I V characteristics became worse with increasing Cl 2 flow rate. The R SC values were found to be , , and Ωcm 2 for the N-face n-gan samples etched using BCl 3 /Cl 2 flow rates of 35/5, 25/15, and 5/35 sccm, respectively. This increasing contact resistance with increasing Cl 2 flow rate is thought to be due to the increasing numbers of ion bombardments during the ICP etching process. With high Cl 2 flow rates, the surfaces might have become damaged due to too many ion bombardments, as shown in Fig. 1(d). The I V characteristics, thus, became worse. The different Ohmic behavior of the ICP-etched N-face n-gan samples may also be associated with the different surface conditions of the etched n-gan layers. This is described in further detail later. To investigate the effect of the Cl 2 flow rate on the surface atomic composition and the surface Fermi level of each of the ICP-etched N-face n-gan samples, we carried out X-ray photoelectron spectroscopy (XPS) with a Mg KαX-ray source in an ultrahigh vacuum chamber. When the Ga/N ratio of the as-grown N-face n-gan sample was set as 1, the Ga/N ratios of the samples etched using BCl 3 /Cl 2 flow rates of 35/5, 25/15, and 5/35 sccm were determined to be 0.85, 0.72, and 0.53, respectively. This means that increasing Cl 2 flow rate during the ICP etching process produces more gallium vacancies (V Ga ) near the surface of the N-face n-gan samples. In addition, Fig. 3 shows that the Ga3d core Fig. 4. AFM images of (a) N-face n-gan ICP etched using BCl 3/Cl 2 with a flow rate of 35/5 sccm, (b) N-face n-gan ICP etched using BCl 3/Cl 2 with a flow rate of 35/5 sccm, followed by KOH etching, (c) N-face n-gan ICP etched using BCl 3/Cl 2 with a flow rate of 5/35 sccm, and (d) N-face n- GaN ICP etched using BCl 3/Cl 2 with a flow rate of 5/35 sccm, followed by KOH etching. level of each of the ICP-etched N-face n-gan samples shifted toward low-binding energy compared with that of the as-grown N-face n-gan. The shift of the surface Fermi level toward low-binding energy is well known to indicate a shift of the Fermi level toward the valence band maximum (VBM) edge [18,19]. The shift of the surface Fermi level in this direction indicates an increase in the surface band bending coincident with an increase in the surface barrier height for the ICP-etched samples. The increased surface barrier could result in increased contact resistance. These results are very consistent with the I V characteristics shown in Fig. 2. Therefore, we can conclude that the plasma damage caused by increasing Cl 2 flow rate during the ICP etching process has brought about a deterioration of the N-face n-gan surfaces. This results in a shift of the surface Fermi level toward low-binding energy, which indicates an increase in the contact resistance. Figures 4(b) and 4(d) show AFM images of the KOHetched samples. We found that the n-gan surface roughness of the ICP-etched sample, shown in Fig. 4(a), was dramatically increased after the KOH etching process (see Fig. 4(b)); the rms value was increased from 5 nm to 150 nm. However, the surface roughness of the KOH-etched sample shown in Fig. 4(d) was not clearly changed compared to that of the ICP-etched only sample shown in Fig. 4(c); the rms values for the samples shown in Figs. 4(c) and (d) were 310 nm and 300 nm, respectively. In addition, the KOH etching depths of both samples were 500 and 250 nm, respectively. Therefore, the KOH etch rate is confirmed to be greatly influenced by the previous ICP etching conditions. The etch rates were 100 and 50 nm/min for the samples previously ICP etched using BCl 3 /Cl 2 flow rates of 35/5 and 5/35 sccm, respectively. The reason for the different etch rates is not

4 Surface and Electrical Properties of Inductively-coupled Tak Jeong et al Fig. 5. I-V curves of the Cr/Ni/Au contacts on the ICPetched N-face n-gan before and after KOH etching. Fig. 6. XPS spectra of the Ga3d core level for the ICPetched N-face n-gan before and after KOH etching. clear, but it may be related to the surface roughnesses of the ICP-etched samples. The I V characteristics of the Cr/Ni/Au Ohmic contacts on the KOH-etched N-face n-gan samples are displayed in Fig. 5. It is clear that the Ohmic behavior of the KOH-etched samples was much improved compared to that of the samples that were only ICP etched. This could be attributed to the removal of the plasma-damaged layer by the KOH etching process. The R SC values for the KOH-etched samples that were previously ICP etched using BCl 3 /Cl 2 with flow rates of 35/5 and 5/35 sccm were found to be and Ωcm 2, respectively. On the other hand, the KOH-etched sample that was previously ICP etched using BCl 3 /Cl 2 with a flow rate of 5/35 sccm still exhibited a higher contact resistance than the as-grown sample did. By comparison, the contact resistance of the KOHetched sample previously ICP etched using BCl 3 /Cl 2 with a flow rate of 35/5 sccm was almost restored to that of the as-grown sample. We suspect that this is mainly attributed to an insufficient etching depth, which results in plasma-damaged layers remaining on the KOHetched n-gan surfaces. Therefore, the surface roughness on the ICP-etched N-face n-gan has to be considered as important in determining the contact resistance. In order to explain the effects of the KOH etching, we used XPS to study the surface atomic compositions and the surface Fermi levels of the KOH-etched N-face n-gan samples, as shown in Fig. 6. When the Ga/N ratio of the as-grown N-face n-gan sample was set as 1, the Ga/N ratio of the KOH-etched sample that was previously ICP etched using BCl 3 /Cl 2 with a flow rate of 35/5 sccm was 1.1. The Ga/N ratio of the sample that was only ICP etched using the same conditions was By comparison, the Ga/N ratio of the KOH-etched sample that was previously ICP etched with BCl 3 /Cl 2 with a flow rate of 5/35 sccm was found to be 0.8 (the ratio was 0.53 for the sample that was only ICP etched using the same conditions). These results are very consistent with the the I V characteristics shown in Fig. 5 and imply that the KOH-etching process produces nitrogen vacancies (V N ), resulting in increased electron conductivity at the n-gan surface. In addition, the Ga 3d peak of sample(b), which was only ICP etched using a BCl 3 /Cl 2 flow rate of 35/5 sccm, shifted toward high-binding energy by an energy of 0.15 ev after the KOH-etching process, as shown in Fig. 6(c). By comparison, the Ga 3d peak of sample(d), which was only ICP etched at a BCl 3 /Cl 2 flow of 35/5 sccm, shifted toward high-binding energy by an energy of 0.47 ev. This indicates that the surface Fermi level moves toward the conduction band minimum (CBM) edge, decreasing the surface barrier height and the surface band bending, which should lower the resistance in the contacts [18, 19]. We can, therefore, conclude that the plasma damage caused by the ICP etching is removed by using the KOH-etching process. This results in a shift of the surface Fermi level toward high-binding energy, which indicates a decrease in the contact resistance. III. CONCLUSIONS We have investigated the surface and the electrical properties of ICP-etched N-face n-gan. Furthermore, a method of reducing the Ohmic contact resistance on the plasma-damaged N-face n-gan has been proposed. We confirmed that the plasma damage caused by the ICP etching brought about a deterioration in the N-face n- GaN surfaces, thereby resulting in a shift of the surface Fermi level toward low-binding energy. This indicates an increase in the contact resistance. We also found that the Ohmic characteristics of the ICP-etched N-face n-gan depended considerably on the etched surface condition of the n-gan. Therefore, the surface roughness on the ICP-

5 Journal of the Korean Physical Society, Vol. 55, No. 3, September 2009 etched N-face n-gan has to be considered as important when determining the contact resistance. However, after the KOH-etching process, the I-V characteristics of the plasma-damaged N-face n-gan were perfectly restored to those of the as-grown sample. This is attributed to the removal of the plasma-damaged layer on the surface of the ICP-etched N-face n-gan, resulting in changes in the surface atomic composition and in the surface Fermi level. ACKNOWLEDGMENTS This work was supported by the Industrial Technology Project in Ministry of Knowledge Economy, Korea, under grunt REFERENCES [1] J. J Wierer, D. A. Steigerwald, M. R. Krames, J. J. O Shea, M. J. Ludowise, G. Christenson, Y.-C. Shen, C. Lowery, P. S. Martin, S. Sbbramanya, W. Gotz, N. F. Garder, R. S. Kern and S. A. Stockman, Appl. Phys. Lett. 78, 3379 (2001). [2] T. Fujii, Y. Gao, R. Sharma, E. L. Hu, S. P. DenBaars and S. Nakamura, Appl. Phys. Lett. 84, 855 (2004). [3] J. K. Kim, J.-Q. Xi, H. Luo and E. F. Schubert, Appl. Phys. Lett. 89, (2006). [4] H. Y. Lee, J. Korean Phys. Soc. 49, 1450 (2006). [5] J. K. Kim, S. I. Na, G. Y. Ha, M. K. Kwon, I. K. Park, J. H. Lim and S. J. Park, Appl. Phys. Lett. 88, (2006). [6] H. S. Kim, K. H. Baik, J. H. Cho, J. W. Lee, S. H. Yoon, H. K. Kim, S. N. Lee, C. S. Sone, Y. J. Park and T. Y. Seong, IEEE Photon. Technol. Lett. 19, 336 (2007). [7] J. K. Lee, H. Cho, B. H. Kim and S. H. Park, J. Korean Phys. Soc. 49, 407 (2006). [8] T. G. Kim, B. H. Kim, I. Y. Kang, Y. C. Chung, J. Ahn, S. Y. Lee, I. S. Park, C. Y. Kim and U. E. Lee, J. Korean Phys. Soc. 49, 721 (2006). [9] H. S. Kim, K. K. Kim, K. K. Choi, H. K. Kim, J. O. Song, J. H. Cho, K. H. Baik, C. S. Sone, Y. J. Park and T. Y. Seong, Appl. Phys. Lett. 91, (2007). [10] C. C. Kao, H. C. Kuo, K. F. Yeh, J. T. Chu, W. L. Peng, H. W. Huang, T. C. Lu and S. C. Wang, IEEE Photon. Technol. Lett. 19, 849 (2007). [11] S. H. Su, C. C. Hou, M. H. Lin, M. Yokoyama, S. M. Chen and H. Kuan, Jpn. J. Appl. Phys. 46, 965 (2007). [12] S. L. Chen, S. J. Wang, K. M. Uang, T. M. Chen and B. W. Liou, IEEE Photon. Technol. Lett. 19, 351 (2007). [13] Y. S. Wu, J. H. Cheng and W. C. Peng, Appl. Phys. Lett. 90, (2007). [14] I. Waki, M. Iza, J. S. Speck, S. P. DenBaars and S. Nakamura, Jpn. J. Appl. Phys. 45, 720 (2006). [15] S. M. Kim, Y. M. Yu, J. H. Baek, S. R. Jeon, H. J. Ahn and J. S. Jang, J. Electrochem. Soc. 154, H384 (2007). [16] J. M. Lee, C. Huh, D. J. Kim and S. J. Park, Semicond. Sci. Technol. 18, 530 (2003). [17] Y. B. Hahn, R. J. Chio, J. H. Hong, H. J. Park, C. S. Choi and H. J. Lee, J. Appl. Phys. 92, 1189 (2002). [18] H. W. Jang, J. H. Lee and J. L. Lee, Appl. Phys. Lett. 80, 3955 (2002). [19] K. A. Rickert, A. B. Ellis, F. J. Himpsel, J. Sun and T. F. Kuech, Appl. Phys. Lett. 80, 204 (2002).